The invention relates to a laser rotation rate sensor in which two light beams counterrotate in a polygon equipped with reflectors in its corners, a signal being derived from their frequency difference which depends on the rate of rotation. The sensor comprises a reflector designed and operated as a magnetooptic element so as to suppress lock-in and includes a prism whose base boundary line borders onto a layer of a gyrotropic material, the reflector being designed in such a manner that total reflection occurs.
It is known that laser rotation rate sensors can be used to measure inertial rotation rates in that the difference in frequency between counterpropagating electromagnetic waves is determined. It is further known that this frequency difference disappears at input rotation rates below a certain threshold value and that thus the rotation rate sensor loses its ability to measure low rotation rates. This phenomenon is called the lock-in effect. To avoid the lock-in effect, various measures have been developed which, in principle, are all based on the fact that a zero frequency split is forced onto the ring laser or--in other words--that its operating point is placed at a point outside the lock-in band.
One of these measures is the use of the magnetooptic Kerr effect. In this case, a nonreciprocal (i.e. direction dependent) phase shift is forced onto the light when it is reflected at the interface between two media of which at least one must be gyrotropic.
Thus, a phase shift difference is generated between the counterpropagating waves of such a rotation rate sensor and this phase shift difference leads to the above-mentioned desired zero frequency split according to the following equation: EQU .DELTA..nu.=(.DELTA..phi./2.pi.).multidot.(c/L)
where
.DELTA..nu.=the frequency difference; PA1 .DELTA..phi.=the phase shift difference: PA1 c=the speed of light; PA1 L=the length of the rotational path.
A corresponding arrangement is known from U.S. Pat. No. 4,225,239. In that patent, a magneto-optic metal mirror is inserted in the beam path in addition to the conventional corner mirrors and the beams impinge on that mirror in a grazing manner.
Such mirrors, however, have the drawback of having an insufficient reflection capability. A solution has been prepared in copending application Ser. No. 468,059 filed Feb. 9, 1983 which describes a rotation rate sensor in which total reflection is utilized. This proposed solution will be described with the aid of FIG. 1.
The laser rotation rate sensor shown in FIG. 1 includes an optical amplifier 1, a mirror 2, a partially transmissive mirror 3, a further mirror 4, a beam divider 4a, a reflector 5 and an optical detector 7 for measuring the frequency difference of the waves. The counterrotating laser beams are designated 8 and 9. Reflectors 2, 3 and 5 are designed and arranged in such a manner that the illustrated rotary paths are created. Beam 9 is reflected at mirror 2, partially reflected by mirror 3 and the remaining portion of the beam is returned by reflector 5 to optical amplifier 1. The portion passing through mirror 3 reaches detector 7.
Beam 8 is deflected in reflector 5, is partially deflected to mirror 2 by the partially transmissive mirror 3 and is returned from mirror 2 to the optical amplifier. The portion passing through the partially transmissive mirror is directed by mirror 4 onto beam divider 4a and a part thereof is likewise deflected toward detector 7.
In order to suppress the lock-in effect, reflector 5 is designed in a special manner. It includes a prism 5a having a certain index of refraction n.sub.1 ; the lateral faces 5b of the prism are inclined in such a manner that, due to meeting the Brewster condition, the generated polarized beams (8 or 9, respectively) are not reflected. A layer 5c of a gyrotropic material is applied to the base face 5d of the prism; this material has an index of refraction n.sub.2 which is less than that of the material of the prism 5a. Thus, with an angle of incidence of appropriate size (not shown here) there will occur total reflection of the beams at the interface 5d between prism 5a and layer 5c. Since, moreover, as a consequence of the applied magnetic field perpendicular to the plane of the drawing and not shown, there occurs a Kerr effect, the beams (9 and 8) experience different shifts in phase. This makes it possible to sense even low rotational rates of the arrangement.
The requirement that a gyrotropic material be used which has an index of refraction that is much smaller than that of the prism material, considerably limits the number of usable substances, since generally most gyrotropic substances have a high index of refraction. This makes difficult, to say the least, the location of a substance that additionally has the greatest possible Faraday rotation and the lowest possible absorption.
It is therefore the object of the invention to modify the proposed arrangement to the extent that the requirement for a certain index of refraction of the gyrotropic material is eliminated and the gyrotropic material can be selected primarily with regard to the other requirements.